It seems like a lot of what I've read has suggested that on the S model, doing the 13t front actually increases top speed because the stock motor wont pull the revs up with the 6th gear being so high. Dunno if that carries over to the SF. For me, I'm not interested in a top end over 80mph on this bike, but I am interested in accelerating and torque all over, so even if I lost a few MPH with the reduction, I'd be happy with the benefits of reduction regardless of Mods.

As for which gear to change, on the S you can change the front 1 tooth and keep the same chain, sounds like for a similar change with the rear you'd have to swap chains. That's a no brainer to me, not sure if it carries over to the SF